Analysis of climatic and edaphic variabilities on the microphytobenthic mat characteristics of a riverine mangrove ecosystem along the southwest coast of India

Abstract

Temporal variation in the taxonomic structure of microphytobenthos (MPB) in a riverine mangrove ecosystem was studied along the southwest coast of India. About 122 species of microphytobenthos comprising diatoms (113 species), cyanobacteria (seven species), dinoflagellate (one species), and euglenophyte (one species) were observed, in which pennate diatoms dominated by 92%. Major subclasses of diatoms identified were Bacillariophycidae, Fragilariophycidae, Thalassiosirophycidae, Melosirophycidae, and Coscinodiscophycidae. Average levels of fluvial nutrients estimated in the porewater were NO₃–N (28 ± 19.7 µmol L⁻¹), PO₄–P (3.77 ± 4.6 µmol L⁻¹), and SiO₄–Si (33.12 ± 27.2 µmol L⁻¹). The colonization and persistence of dense cyanobacterial mats during monsoon resulted in less abundance and diversity of diatoms than in other seasons. The numerical abundance of MPB was at its maximum during July (monsoon season) due to the dense cyanobacterial mat formed by Oscillatoria princeps. MPB diversity was at its maximum during MON (H´- 3.2), followed by POM (H´- 3.08) and lowest during PRM (H´- 2.23). The statistically significant seasonal variations in the diversity of MPB were noticed during the study period (ANOVA F value 8.120; df 2; p value < 0.05). The present study identifies sediment temperature and porewater salinity (freshwater preference) along with rainfall, sediment pH and C:N ratio as the major governing factors in the benthic microalgal mat formation of the study area.